Fireflooding is a process for the enhanced recovery of oil from a petroleum reservoir. In a fireflood operation a gaseous oxidant such as air, oxygen-enriched air, or high purity oxygen, is injected into a well that extends into a reservoir. The oxidant reacts in-situ with some of the fuel in the reservoir. This combustion releases heat and produces carbon dioxide as one combustion reaction product. The viscosity of the heavy oil within the reservoir is reduced by the released heat and by dissolution of combustion gases. The pressure and heat front associated with the fireflood operation serve to improve movement of the oil toward production wells and result in increased recovery of the oil.
The selection of air or oxygen for fireflood operations generally depends on the reservoir characteristics. For many applications high purity oxygen is more economical than air because approximately only one fifth the flow rate is required to inject equivalent oxygen thereby reducing compression energy for injection into the reservoir. Also, by using high purity oxygen, the injection of large amounts of nitrogen into the reservoir is avoided thus serving to improve the quality of gaseous fuels or carbon dioxide which may be recovered from the reservoir and reducing the required gas treatment capacity and associated costs.
With oxidant injection by itself, only about one third of the in-situ generated heat serves to improve heavy oil mobility within the reservoir while about two thirds of the in-situ generated heat remains in the burned out portion of the reservoir. This problem has been addressed by injecting a liquid heat transfer medium, e.g. water, into the reservoir along with the oxidant in either an intermittent or continuous manner. The resultant steam adds to the combustion front gas flow and transfers heat to the oil upon its condensation. This serves to carry heat away from the combustion zone of the reservoir and causes improved mobility of the heavy oil with increased thermal efficiency for the fireflood operation.
One problem with this use of water is the possibility of some of the oxidant passing into the water delivery system. The presence of the oxidant within the water delivery system substantially increases the probability of corrosion and thus failure of the equipment. Further, the water injection piping and valves may contain hydrocarbon contaminants. Thus oxidant contact with the water injection string is quite hazardous because of the marked increase in the possibility of fire or explosion. Each of these problems is heightened when high purity oxygen is employed as the oxidant.
It is therefore an object of this invention to provide a process and apparatus for the injection of gaseous oxidant and liquid heat transfer media into a well while preventing oxidant migration into the liquid delivery system.